Solid oxide cells (SOCs) generally include cells designed for different applications, such as solid oxide fuel cells (SOFCs) or solid oxide electrolysis cells (SOECs). Due to their common basic structure, the same cell may, for example, be used in SOFC applications as well as SOEC applications. Since in SOFCs fuel is fed into the cell and converted into power, while in SOECs power is applied to produce fuel, these cells are often referred to as ‘reversible’ SOCs.
Solid oxide cells may have various designs. Typical configurations include an electrolyte layer being sandwiched between two electrodes. During operation of the cell, usually at temperatures of about 500° C. to about 1100° C., one electrode is in contact with oxygen or air, while the other electrode is in contact with a fuel gas.
The most common manufacture processes suggested in the prior art comprise the manufacture of single cells. Generally, a support is provided, on which an electrode layer is formed, followed by the application of an electrolyte layer. The so formed half cell is dried and afterwards sintered, in some cases in a reducing atmosphere. Finally, a second electrode layer is formed thereon so as to obtain a complete cell. Alternatively, one of the electrode layers or the electrolyte layer may be used as a support layer, having a thickness of about 300 μm or more.
This approach usually requires a relatively thick support layer to provide mechanical stability of the obtained cell, thereby increasing the overall thickness of the single cells. It has been suggested to form the support from metals or metal alloys, which are less brittle than ceramic materials and therefore superior in mechanical stability. However, disadvantageously it has been found that due to the metallic materials used, poisoning of the catalyst in the adjacent electrode layer due to migration from the support, especially if chromium is used in the support, may occur. Furthermore, metal supports are not suitable for SOCs intended for high temperature applications in the range up to about 1000° C.
If alternatively one of the electrodes is also used as the support layer, on the one hand the overall thickness of said layer determines the mechanical stability of the cell, i.e. the layer must be sufficiently thick; on the other hand the layer thickness influences the gas diffusion through the electrode layer and should therefore be sufficiently thin. Furthermore, in order to produce cells as cost effective as possible, the amount of materials used for each layer should be kept to a minimum.
US-A-2004/0166380 (Gorte et al) relates to porous electrodes for use in SOFCs, wherein the electrodes are comprised primarily of a ceramic material and an electrochemically conductive material. The electrodes are prepared by impregnating a porous ceramic material with precursors of the electrochemically conducting material. The focus is especially on providing a cathode comprising a porous ceramic matrix and an electrochemically conducting material dispersed at least partially within the porous ceramic matrix, wherein the porous ceramic matrix includes a plurality of pores having a pore size of at least about 0.5 μm.
US-A-2004/0018409 (Hui et al) discloses a SOFC comprising a dense electrolyte disposed between a porous anode and a porous cathode. The electrolyte may preferably be yttria stabilized zirconia. The anode may be formed from yttrium-doped strontium titanate, yttrium-doped strontium titanate and nickel, doped ceria, lanthanum-doped ceria and nickel or yttria stabilized zirconia and nickel. The cathode may be formed from strontium-doped lanthanum manganite or doped lanthanum ferrite. The SOFC may further comprise ‘interlayers’ disposed between the electrodes and the electrolyte. Said layers are dense layers which function as a barrier layer. The interlayers further do not comprise any catalyst material, and since the layers are dense layers, they cannot function as electrodes.
WO-A-2006/082057 (Larsen) relates to a SOFC comprising an electrolyte layer sandwiched in between two electrode layers, and further a metallic support for mechanical stability of the cell.
US-A-2004/0101729 (Kearl) relates to a SOFC with a thin film electrolyte in combination with both, a thick film anode/fuel electrode and a thick film cathode/air electrode. The cathode preferably comprises a material, such as silver, or a material having a perovskite structure, such as lanthanum strontium manganite, lanthanum strontium ferrite, lanthanum strontium cobaltite, LaFeO3/LaCoO3, YMnO3, CaMnO3, YfeO3, and mixtures thereof. The cell may further comprise interfacial layers between the electrodes and the electrolyte layer. Said interfacial layers do not comprise any catalyst material, and since the layers are dense layers, they cannot function as electrodes.
WO-A-98/49738 (Wallin et al) discloses a composite oxygen electrode/electrolyte structure for a solid state electrochemical device having a porous composite electrode in contact with a dense electrolyte membrane, said electrode comprising:                (a) a porous structure having interpenetrating networks of an ionically-conductive material and an electronically-conductive material; and        (b) an electrocatalyst different from the electronically-conductive material, dispersed within the pores of the porous structure.        
WO-A-2007/011894 (Hertz et al) discloses a thin-film composite material with nanometer-scale grains, comprising a thin-film layer that includes:                a) an electronic conductor; and        b) an ionic conductor.        
US-A-2003/0082436 (Hong et al) relates to an electrode for a SOFC, sensor or solid state device, comprising an electrode coated with an oxygen ion conducting ceramic ceria film. The electrolyte may be a YSZ electrolyte sandwiched by Pt-LSM electrodes.
U.S. Pat. No. 5,543,239 (Virkar et al) discloses an improved electrode/electrolyte structure having an enhanced three-faced boundary length for use as a fuel cell, a catalyst or a sensor, wherein said structure comprises:                a) a substrate layer consisting of the dense electrolyte material;        b) a porous surface layer of said dense electrolyte material over the dense electrolyte substrate layer;        c) an electrocatalyst material on and within the porous surface layer of electrolyte, wherein the electrocatalyst material is continuous on the surface of the porous electrolyte, creating enhanced three-faced boundaries with gas present; and        d) said structure is integrally connected or attached to a porous anode.        
US-A-2006/0093884 (Seabaugh et al) relates to a ceramic laminate structure including partially stabilized zirconia electrode layers, sandwiching a fully stabilized zirconia electrolyte layer.
US-A-2008/0038611 (Sprenkle et al) discloses an electrode supported electrolyte membrane for an electrochemical cell comprising:                a substantially continuous layer of a ceramic ion conducting electrolyte supported on a conductive electrode substrate, wherein the substrate includes an active electrode layer and a bulk electrode layer;        a backing structure on a face of the bulk electrode layer opposite the electrolyte layer with a thermal expansion coefficient approximately equal to the thermal expansion coefficient of the electrolyte layer.        
EP-A-1482584 (Komada et al) teaches an electrode for a solid oxide cell wherein:                the electrode comprises a skeleton constituted of a porous sintered compact having a three dimensional network structure, the porous sintered compact being made of an oxide ion conducting material and/or a mixed oxide ion conducting material;        grains made of an electron conducting material and/or a mixed oxide ion conducting material are adhered onto the surface of said skeleton; and        said grains are baked inside the voids of said porous sintered compact under the conditions such that the grains are filled inside the voids.        
In view of the disadvantages of the SOC compositions of the prior art, there is still a desire for improved SOCs which are durable, have good mechanical stability, do not suffer from the above described drawbacks of the SOCs of the prior art, may be used in a wide temperature range up to 1000° C. or above, and which have an overall excellent life time.